When the MXA wrecking crew was
snooping around the KTM factory, we
spotted this prototype aluminum KTM
frame in the corner of the R&D department. They tested it and didn’t like it.

ALUMINUM VERSUS STEEL

Dear MXA,I thought the major reason mostmanufacturers switched to aluminumframes was to reduce weight, andyet the 2018 KTM 450SXF is 9 to

19 pounds lighter than its competitors, some of which don’t even have
electric start. Is it time for the other
manufacturers to rethink steel?

First, all of the KTM’s weight savings over the Honda, Yamaha, Suzuki
and Kawasaki are not a result of the
weight difference between aluminum
and steel. KTM spent a lot of years,
time and money casting smaller
and more compact engines to save
weight (and that also includes a

1-pound-lighter 2019 engine). As anyracer who ever tried to knock weightoff his race bike knows, no singlecomponent is going to get the jobdone. It requires an overall approachthat worries not just about poundsand ounces on the frame, suspensionand engine, but grams on virtuallyevery component. As far as the differ-ence between KTM’s chromoly steelframe and the aluminum frames ofthe “Big Four” Japanese brands, thereare lots of market forces at work.

Here is a quick list.

(1) Brooklyn Bridge. Originally,
aluminum frames were lighter than
the steel frames they replaced, but
over time, they began to gain weight.
As tubes were replaced with castings
and, eventually, castings with forgings, the aluminum frames became
stronger, more rigid and offered
increased durability. But, they got
heavier. Engineers knew this as far
back as the 1990s. In fact, in the
1990s, ATK designer Horst Leitner
got tired of beefing up the aluminum
swingarms that came on ATK 604s
and welded up a chromoly swingarm
of the same dimensions. Shockingly,
he discovered that the old-fashioned
steel swingarm was several pounds
lighter. The days of the simple aluminum swingarm had been replaced by
Brooklyn Bridge-style structures that
were no longer lighter than a steel
swingarm. KTM stuck with a chromoly frame because its test riders
preferred the feel, but if you look left,
you will see a prototype KTM aluminum frame that they built to test the
feasibility of going to aluminum. They
decided not to.

( 2) Cost factor. Making a chromoly frame is a time-consuming and
expensive venture. The chromoly
tubes have to be bent, mitered,
gussetted, welded, heat treated and
cold set. There are many more steps
required to make a classic steel frame
than an aluminum frame. But, chromoly leaves lots of R&D leeway to
make quick and simple test frames.
Aluminum frames are much cheap-er to build (once you amortize the
casting costs). The aluminum pieces
simply plug into each other where
they can be welded on a jig. They
don’t need immediate heat treating
because they achieve their strength
as they travel across the oceans.
With as few as 10 aluminum pieces, no bending and no mitering or
gusseting, mass-producing aluminum
frames is a money- and time-saver.

( 3) Steel future. If we were
betting men, we’d bet the house
that there will be no steel-framed
Japanese motocross bikes in the
future.